1. Field of the Invention
The present invention relates to an acoustic resonator and a filter, and more particularly to an acoustic resonator capable of suppressing the occurrence of spurious components, and a filter using the same.
2. Description of the Background Art
There is a demand for reducing the size and weight of components to be used in electronic devices such as portable devices. For example, a filter used in a portable device is required to be small in size and to allow for precise adjustment of the frequency characteristics thereof. A filter known in the art that satisfies such a requirement is a filter using an acoustic resonator as disclosed in Japanese Laid-Open Patent Publication No. 60-68711 (Patent Document 1).
A conventional acoustic resonator will now be described with reference to FIG. 17A to FIG. 17D.
FIG. 17A is a cross-sectional view showing the basic structure of a conventional acoustic resonator 500. The acoustic resonator 500 includes a piezoelectric element 501 interposed between an upper electrode 502 and a lower electrode 503. The acoustic resonator 500 is used while being placed on a substrate 505 with a cavity 504 formed therein. The cavity 504 can be formed by partially etching the substrate 505 from the bottom surface thereof using a micromachining method. The upper electrode 502 and the lower electrode 503 apply an electric field across the acoustic resonator 500 in the thickness direction, thereby causing the acoustic resonator 500 to vibrate in the thickness direction. Next, the operation of the acoustic resonator 500 will be described with respect to the thickness longitudinal vibration of an infinite flat plate.
FIG. 17B is a schematic perspective view illustrating the operation of the conventional acoustic resonator 500. When an electric field is applied across the acoustic resonator 500 between the upper electrode 502 and the lower electrode 503, an electric energy is converted to a mechanical energy by the piezoelectric element 501. The induced mechanical vibration is a thickness extensional vibration, and causes the piezoelectric element 501 to expand and contract in the same direction as the electric field. Typically, the acoustic resonator 500 utilizes the resonant vibration in the thickness direction of the piezoelectric element 501 to resonate at a frequency such that the thickness thereof is equal to the half wavelength. The cavity 504 shown in FIG. 17A is used for accommodating the thickness longitudinal vibration of the piezoelectric element 501.
As shown in FIG. 17D, the equivalent circuit of the acoustic resonator 500 has both a series resonance portion and a parallel resonance portion. The equivalent circuit includes a series resonance section made up of a capacitor C1, an inductor L1 and a resistor R1, and a capacitor C0 connected in parallel to the series resonance section. With such a circuit configuration, the equivalent circuit has admittance-frequency characteristics with a maximum admittance at the resonance frequency fr and a minimum admittance at the antiresonance frequency fa, as shown in FIG. 17C. The resonance frequency fr and the antiresonance frequency fa satisfy the following relationship.fr=1(2π√{square root over (L1×C1)})fa=fr√{square root over (1+C1/C0)}
If the acoustic resonator 500 having such admittance-frequency characteristics is used as a filter, it is possible to realize a filter having a small size and a small loss because of the utilization of the resonant vibration of the piezoelectric element 501.
Another conventional acoustic resonator as disclosed in Japanese Laid-Open Patent Publication No. 2003-87085 (Patent Document 2), for example, aims at reducing the energy loss at the electrode while improving the resonance frequency stability against temperature changes. FIG. 18 is a cross-sectional view showing the basic structure of a conventional acoustic resonator 510 disclosed in Patent Document 2. The conventional acoustic resonator 510 includes a substrate 515 with a cavity 514 formed therein, and a support layer 513 formed on the substrate 515. A lower electrode 513 is formed on a support layer 516. A piezoelectric element 511 is formed on the lower electrode 513. An upper electrode 512 is formed on the piezoelectric element 511.
The conventional acoustic resonator 510 is designed so that the thickness of the support layer 516 is equal to the thickness of a vibrating section including the piezoelectric element 511, the upper electrode 512 and the lower electrode 513, i.e., so that the interface between the lower electrode 513 and the support layer 516 coincides with an antinode of displacement distribution caused by an nth harmonic wave. With such a configuration, the conventional acoustic resonator 510 aims at reducing the vibration loss at the electrode section.
Since only a portion of an acoustic resonator is actually fixed to a substrate, not the whole of the acoustic resonator will freely vibrate in the thickness longitudinal mode. Referring to FIG. 17A, a vibrating section is divided into a first region around the cavity that vibrates while being fixed, and a second region within the opening of the cavity that vibrates with opposite ends thereof being free ends. In the vibrating section, a vibration that is dictated by the thickness of the vibrating section is caused as the primary resonant vibration. However, the vibration at a frequency f1 caused in the vibrating section may leak to the substrate in the fixed region around the cavity. This phenomenon occurs because the portion around the cavity to which the vibrating section is fixed is not actually serving as a completely fixed end. Therefore, the transverse mode vibration occurring in the vibrating section propagates to the substrate via the fixed portion, thus causing the vibration leak. As a result, the leaking vibration induces other vibrations, whereby a spurious vibration occurs near the primary resonant vibration (f1), in addition to the intended thickness extensional vibration of the fundamental mode (½ wavelength mode, the frequency f1). This means that a portion of the energy to be used for causing a vibration inside the piezoelectric element is lost by the vibration leak.
Such a spurious vibration is caused due to the vibration leak to the substrate via the fixed portion. For example, where the resonance frequency of the vibration mode dictated by the cavity depth is present near the primary resonance frequency, a vibration dictated by the cavity depth occurs as a spurious vibration near the resonance frequency of the vibrating section due to the vibration leak. If the frequency of the spurious vibration, is between the resonance frequency fr and the antiresonance frequency fa, a spurious component appears as shown in FIG. 19A. If acoustic resonators having such a spurious vibration are connected in parallel as shown in FIG. 19B to form a filter, undesirable bandpass characteristics appear in the passband portion as shown in FIG. 19C. The bandpass characteristics lower the communication quality.
However, the conventional acoustic resonators disclosed in Patent Document 1 and Patent Document 2 fail to take into consideration the spurious vibration occurring due to the vibration leak from the support section to the substrate, i.e., the spurious vibration dictated by the cavity depth. With these conventional techniques, it is not possible to realize an acoustic resonator having admittance-frequency characteristics with no spurious components.